Internal Field Emission in Siliconp−nJunctions

Internal field emission is believed to occur in narrow $p-n$ junctions in silicon. This conclusion is based on the following observations: (i) as in wide junctions, the multiplication characteristic has a positive temperature coefficient whereas the reverse characteristic, unlike wide junctions, possesses a negative temperature coefficient; (ii) the forward and reverse currents are relatively insensitive to temperature; (iii) radiative transitions of energetic carriers in the high-field region result in the emission of visible light, the pattern of which shows that the current flows more or less uniformly through the whole extent of the junction but that the array of fewer and more intense light spots characteristic of avalanche breakdown is absent; (iv) the noise associated with the onset of avalanche breakdown is absent. It is concluded that in these narrow junctions the junction space charge field due to the built-in potential alone is sufficient to result in a rate of carrier generation much greater than that of the normal thermal processes. By assuming a reasonable form for the voltage dependence of the field-generated current, it proves possible to account qualitatively for the complex forward-bias characteristics of these junctions. The reverse characteristics show the onset of multiplication of field-emitted carriers at a reasonable threshold. Possible causes of the softness of the reverse characteristics are discussed.